RCD keeps tripping all MCBs are off

ra9jd

I have a fuse box which has been split to have two sets of MCBs with one RCD each. One is for the appliances/plugs in the property, and the second for the lights.

For some reason, the RCD for the lights has tripped but all MCBs are still on. I have switched all MCBs off to test, and the RCD is still switching itself off.

Constantly testing, at times, the RCD does come on, but trips itself after a minute or so. This is whether the MCBs are up or down. 

Any clue on why this could be happening and how to fix it? 

47Revisited

Loose wire in light fitting or switch.

dil1976

47Revisited said:

Loose wire in light fitting or switch.

Thats a highly unlikely reason for a RCD to trip.


OP you need to start by unplugging everything and seeing if one particular item is causing it too trip as it wont trip for no reason, if you cant deduct it with that method then its time to call in professional help

47Revisited

dil1976 said:

47Revisited said:

Loose wire in light fitting or switch.

Thats a highly unlikely reason for a RCD to trip.


OP you need to start by unplugging everything and seeing if one particular item is causing it too trip as it wont trip for no reason, if you cant deduct it with that method then its time to call in professional help

I disagree! 

Op said RCD for the lights. Unplugging everything will lead nowhere. unless he has borrowed neutrals/or any other wire.

Who knows perhaps someone's even hung a picture in the wrong place..but normally a loose wire 

nofoollikeold

The MCBs will only be in the live, so could be a neutral to ground fault.

fenwick458

loose wires don't normally cause RCD's to trip.

47Revisited

Other possibilities a leak or faulty lighting/ switch accessories.

grumbler

RCD can be tripped by a neutral wire touching earth.

But if all MCBs are off it seems to be no current to trip the RCD.

Can it be that some current gets to the sockets neutral from lighting neutral (and then to the earth thus tripping the RCD)?

Section62

grumbler said:

RCD can be tripped by a neutral wire touching earth.

But if all MCBs are off it seems to be no current to trip the RCD.

Current flowing on the neutral side to 'earth' would create an inbalance across the RCD and is enough to make it trip (in the right circumstances).

dil1976

47Revisited said:

dil1976 said:

47Revisited said:

Loose wire in light fitting or switch.

Thats a highly unlikely reason for a RCD to trip.


OP you need to start by unplugging everything and seeing if one particular item is causing it too trip as it wont trip for no reason, if you cant deduct it with that method then its time to call in professional help

I disagree! 

Op said RCD for the lights. Unplugging everything will lead nowhere. unless he has borrowed neutrals/or any other wire.

Who knows perhaps someone's even hung a picture in the wrong place..but normally a loose wire 

Be interesting to know how the OP knows it the lights that are tripping the RCD!

Even so a loose connection is highly unlikely to cause this to happen

grumbler

Section62 said:

grumbler said:

RCD can be tripped by a neutral wire touching earth.

But if all MCBs are off it seems to be no current to trip the RCD.

Current flowing on the neutral side to 'earth' would create an inbalance across the RCD and is enough to make it trip (in the right circumstances).

Yes, this is obvious.

The question was in the last sentence of my post that you excluded from the quote - what can cause this current when all live wires are disconnected by MCBs.

ra9jd, switch all MCBs off, including lighting, and I bet the RCD will stop tripping. In this case the fault is a neutral wire touching earth somewhere in the sockets circuits. If you feel confident enough you can find out which circuit exactly by disconnecting one by one neutral wires in the CU .